How Much Sleep Do You Need?

Alright, so sleep is important, but how much sleep do you really need? To answer that question, let's consider an experiment conducted by researchers at the University of Pennsylvania and Washington State University.

The researchers began the experiment by gathering 48 healthy men and women who had been averaging seven to eight hours of sleep per night. Then, they split these subjects into four groups. The first group had to stay up for 3 days straight without sleeping. 

The second group slept for 4 hours per night. The third group slept for 6 hours per night. And the fourth group slept for 8 hours per night. In these final three groups—4, 6, and 8 hours of sleep—the subjects were held to these sleep patterns for two weeks straight. Throughout the experiment the subjects were tested on their physical and mental performance.

Here's what happened…

The subjects who were allowed a full 8 hours of sleep displayed no cognitive decreases, attention lapses, or motor skill declines during the 14-day study. Meanwhile, the groups who received 4 hours and 6 hours of sleep steadily declined with each passing day. The four-hour group performed worst, but the six-hour group didn't fare much better. In particular, there were two notable findings.

First, sleep debt is a cumulative issue. In the words of the researchers

, sleep debt “has a neurobiological cost which accumulates over time.” After one week, 25 percent of the six-hour group was falling asleep at random times throughout the day. After two weeks, the six-hour group had performance deficits that were the same as if they had stayed up for two days straight. Let me repeat that: if you get 6 hours of sleep per night for two weeks straight, your mental and physical performance declines to the same level as if you had stayed awake for 48 hours straight.

Second, participants didn't notice their own performance declines. 

When participants graded themselves, they believed that their performance declined for a few days and then tapered off. In reality, they were continuing to get worse with each day. In other words, we are poor judges of our own performance decreases even as we are going through them.